1. Technical Field
The invention generally relates to sound processing systems. More particularly, the invention relates to sound processing systems having multiple outputs.
2. Related Art
Consumer expectations of sound quality in audio or sound systems are increasing. In general, such consumer expectations have increased dramatically over the last decade, and consumers now expect high quality sound systems in a wide variety of listening environments, including vehicles. In addition, the number of potential audio sources has increased. Audio is available from sources such as radio, compact disc (CD), digital video disc (DVD), super audio compact disc (SACD), tape players, and the like. While sound systems have traditionally supported two-channel (“stereo”) formats, today many sound systems include surround processing systems that create a perception that sound is coming from all directions around a listener (a “surround effect”). Such surround sound systems may support formats using more than two discrete channels (“multi-channel surround systems”). Creation of the surround effect in a wide variety of listening environments requires consideration of a different set of variables depending on the listening environment.
Surround sound systems generally use three or more loudspeakers (also referred to as “speakers”) that reproduce sound from two or more discrete channels to create the surround effect. Successful development of the surround effect involves creating a sense of envelopment and spaciousness. Such a sense of envelopment and spaciousness, while very complex, generally depends on the spatial properties of the background stream of the sound being reproduced. Reflective surfaces aid the sense of envelopment and spaciousness in the listening environment because reflective surfaces redirect impacting sound back towards the listener. The listener may perceive this redirected sound as originating from the reflective surface or surfaces, thus creating the perception that the sound is coming from all around the listener is enhanced.
Many digital sound processing formats support direct encoding and playback of sounds using multi-channel surround processing systems. Some multi-channel surround processing systems have five or more channels, where each channel carries a signal for conversion into sound waves by one or more loudspeakers. Other channels, such as a separate band limited low frequency channel, also may be included. A common multi-channel surround processing format (referred to as a “5.1 system”) uses five discrete channels and an additional band limited low frequency channel that generally is reserved for low frequency effects (“LFE”). Recordings made for reproduction by 5.1 systems may be processed with the assumption that the listener is located at the center of an array of loudspeakers that includes three speakers in front of the listener and two speakers located somewhere between and including the sides of the listener and about 45 degrees behind the listener. In five channel multi-channel surround systems, both the channels and the signals carried by the channels may be referred to as left-front (“LF”), center (“CTR”), and right-front (“RF”), left-surround (“LSur”), and right-surround (“RSur”). When seven channels are implemented, LSur and RSur may be replaced by left-side (“LS”), right-side (“RS”), left-rear (“LR”) and right-rear (“RR”).
Most recorded material is provided in traditional two-channel stereo. However, a surround effect can be achieved from two-channel signals through the use of matrix decoders. Matrix decoders may synthesize four or more output signals or outputs from two input signals, which may include a left input signal and a right input signal. When used in this manner, matrix decoders mathematically describe or represent various combinations of input signals in an N×2 or other matrix, where N is the number of desired outputs. In a similar manner, matrix decoders may also be used to synthesize additional output signals from three or more discrete input signals using an N×M matrix, where M is the number of discrete input channels.
When used to create a surround effect from a two-channel signal, a matrix usually includes 2N matrix coefficients that define the proportion of the left and/or right input signals for a particular output signal. The values of the matrix coefficients generally depend, in part, on the intended direction of the recorded material as indicated by one or more steering angles. Each steering angle may be a function of two signals. In general, one steering angle is a function of the left and right input signals (the “left/right steering angle” or “lr”), and another steering angle is a function of two signals derived from the right and left input signals (the “center/surround steering angle” or “cs”). Each steering angle indicates the intended direction of the recorded material in terms of an angle between the two signals from which it was derived.
The design of audio or sound systems involves the consideration of many different factors, including for example, the position and number of speakers and the frequency response of each speaker. The frequency response of most speakers traditionally has been limited such that many speakers cannot reproduce low frequencies accurately, if at all. Therefore, most surround processing systems also include a separate speaker or speakers designed and dedicated to producing these low frequency signals. To direct the low frequency signals to this separate low frequency speaker, surround sound systems may employ a process known as “bass management.” Traditional bass management separates the low frequencies from each channel using a crossover filter and adds them together to create a single channel (“mono”) signal. This procedure may lead to degradation of the surround effect because the combined low frequencies are not decorrelated. Unfortunately, foregoing the traditional bass management may also lead to undesirable results because the low frequencies sound quite unnatural when steered by most matrix decoders.
In another example, the physical properties of a listening environment and/or the manner in which a listening environment will be used dictate the factors that need to be considered when designing sound systems. Most surround sound systems are designed for optimum listening environments. Optimum listening environments generally are reverberant and center the listener among an array of speakers, facing forward in a position known as the “sweet spot.” However, the physical properties of non-optimum listening environments can be much different and generally require that different factors be considered when sound systems are designed. One example includes, listening environments that are enjoyed simultaneously by more than one listener, none of whom may be stationary or located in the “sweet spot.” Another example includes, listening environments that are quite small and are not very reflective. Such listening environments present a challenge in creating the surround effect. In yet a further example, the listening environment may be such that the listener or listeners are located near one or more of the speakers. Most surround sound systems were simply not designed with these factors in mind.
A vehicle is an example of a non-optimum listening environment in which listener placement, speaker placement and lack of reflectivity are important factors in the design of surround sound systems for that listening environment. A vehicle may be more confined than rooms containing home theatre systems and much less reflective. In addition, the speakers may be in relatively close proximity to the listeners and there may be less freedom with regard to speaker placement in relation to the listener. In fact, it may be nearly impossible to place each speaker the same distance from any of the listeners. For example, in an automobile, the front and rear seating positions and their close proximity to the doors, as well as the size and location of kick-panels, the dash, pillars, and other interior vehicle surfaces that could contain the speakers all serve to limit speaker placement. In another example, when the center speaker is placed in the dash, the size of the center speaker is limited due to the space constraints within the dash. These placement and size restrictions are problematic considering the short distances available in an automobile for sound to disperse before reaching the listeners or the walls. Due to these factors, multi-channel surround processing systems suffer serious quality degradation when implemented in non-optimum listening environments.